Settings > Other > Close account
misskey.io ではアカウントの閉鎖の API を無効にしているようです。
misskey.io ではないインスタンスでは @AlfredSK の方法でアカウントの閉鎖ができます。
[不明] This question would not be asked if that button is working.
Then you should mention this in your question.
[不明]
Currently, Misskey has a bug where the information is not deleted properly from the remote instance when the account is deleted.
Therefore, you cannot delete accounts with Misskey.io until the bug is resolved.